MLT04 Datasheet, PDF (3/12 Pages) Analog Devices – Four-Channel, Four-Quadrant Analog Multiplier

MLT04
FUNCTIONAL DESCRIPTION
The MLT04 is a low cost quad, 4-quadrant analog multiplier with
single-ended voltage inputs and voltage outputs. The functional
block diagram for each of the multipliers is illustrated in Figure 3.
Due to packaging constraints, access to internal nodes for externally
adjusting scale factor, output offset voltage, or additional summing
signals is not provided.
+V_S
X1, X2, X3, X4
MLT04
G1, G2, G3, G4
0.4
W1, W2, W3, W4
Y1, Y2, Y3, Y4
–V_S
Figure 3. Functional Block Diagram of Each MLT04
Multiplier
Each of the MLT04’s analog multipliers is based on a Gilbert cell
multiplier configuration, a 1.23 V bandgap reference, and a unity-
connected output amplifier. Multiplier scale factor is determined
through a differential pair/trimmable resistor network external to
the core. An equivalent circuit for each of the multipliers is shown
in Figure 4.
V_CC
INTERNAL
BIAS
W
OUT
X_IN
GND
Y_IN
V_EE
22k
22k
22k
SCALE
FACTOR
200µA 200µA 200µA 200µA
200µA 200µA
Figure 4. Equivalent Circuit for the MLT04
Details of each multiplier’s output-stage amplifier are shown in
Figure 5. The output stages idles at 200 µA, and the resistors in
series with the emitters of the output stage are 25 Ω. The output
stage can drive load capacitances up to 500 pF without oscillation.
For loads greater than 500 pF, the outputs of the MLT04 should
be isolated from the load capacitance with a 100 Ω resistor.
V_CC
25Ω
W
OUT
25Ω
ANALOG MULTIPLIER ERROR SOURCES
Multiplier errors consist primarily of input and output offsets, scale
factor errors, and nonlinearity in the multiplying core. An expres-
sion for the output of a real analog multiplier is given by:
V_O = ( K + ∆K ){(V_X + X _OS )(V_Y + Y _OS ) + Z_OS + f ( X , Y )}
where:
K
=
∆K
=
V
=
X
X
=
OS
V
=
Y
Y
=
OS
Z
=
OS
ƒ(X, Y) =
Multiplier Scale Factor
Scale Factor Error
X-Input Signal
X-Input Offset Voltage
Y-Input Signal
Y-Input Offset Voltage
Multiplier Output Offset Voltage
Nonlinearity
Executing the algebra to simplify the above expression yields
expressions for all the errors in an analog multiplier:
Term Description
Dependence on Input
KV V True Product
XY
∆KV_YV_Y
VY
X OS
Scale-Factor Error
Linear “X” Feedthrough
Due to Y-Input Offset
Goes to Zero As Either or
Both Inputs Go to Zero
Goes to Zero at V , V = 0
XY
Proportional to V
X
VX
Y OS
Linear “Y” Feedthrough Proportional to V
Y
Due to X-Input Offset
X Y Output Offset Due to X-, Independent of V , V
OS OS
XY
Y-Input Offsets
Z
Output Offset
OS
ƒ(X, Y) Nonlinearity
Independent of V , V
XY
Depends on Both V , V .
XY
Contains Terms Dependent
on V , V , Their Powers
XY
and Cross Products
As shown in the table, the primary static errors in an analog
multiplier are input offset voltages, output offset voltage, scale
factor, and nonlinearity. Of the four sources of error, only two are
externally trimmable in the MLT04: the X- and Y-input offset
voltages. Output offset voltage in the MLT04 is factory-trimmed to
± 50 mV, and the scale factor is internally adjusted to ± 2.5% of full
scale. Input offset voltage errors can be eliminated by using the
optional trim circuit of Figure 6. This scheme then reduces the net
error to output offset, scale-factor (gain) error, and an irreducible
nonlinearity component in the multiplying core.
+V_S
50kΩ
50kΩ
I
–V_S
±100mV
FOR X_OS, Y_OS TRIM
CONNECT TO SUM
NODE OF AN EXT OP AMP
Figure 6. Optional Offset Voltage Trim Configuration
V_EE
Figure 5. Equivalent Circuit for MLT04 Output Stages
REV. B
–3–